Ferritic stainless steel and process for producing same

Abstract

Ferritic stainless steel that has excellent formability and ridging resistance and can be produced with high productivity is provided. The ferritic stainless steel has: a predetermined chemical composition; a microstructure containing ferrite crystal grains which satisfy at least one of a C concentration of 2C C or more and an N concentration of 2C N or more, the ferrite crystal grains having a volume fraction with respect to a whole volume of the microstructure of 5% or more and 50% or less, where C C and C N are respectively C content and N content in the steel in mass %; and a Vickers hardness of 180 or less.

The invention claimed is: 
     
       1. A ferritic stainless steel comprising:
 a chemical composition consisting of, in mass %, 
 C: 0.005% to 0.050%, 
 Si: 0.01% to 1.00%, 
 Mn: 0.01% to 1.0%, 
 P: 0.040% or less, 
 S: 0.010% or less, 
 Cr: 15.5% to 17.5%, 
 Ni: 0.01% to 1.0%, 
 Al: 0.001% to 0.10%, and 
 N: 0.005% to 0.06%, 
 and optionally
 one or more selected from Cu: 0.01% to 1.0%, Mo: 0.01% to 0.5%, and Co: 
 
 0.01% to 0.5%, 
 and optionally
 one or more selected from V: 0.01% to 0.25%, Ti: 0.001% to 0.10%, Nb: 0.001% to 0.05%, Ca: 0.0002% to 0.0020%, Mg: 0.0002% to 0.0050%, B: 0.0002% to 0.0050%, and REM: 0.01% to 0.10%, 
 
 with a balance being Fe and incidental impurities;
 a microstructure containing ferrite crystal grains which satisfy at least one of a C concentration of 2C C  or more and an N concentration of 2C N  or more, the ferrite crystal grains having a volume fraction with respect to a whole volume of the microstructure of 5% or more and 50% or less, where C C  and C N  are respectively C content and N content in the steel in mass %; and 
 a Vickers hardness of 180 or less. 
 
 
     
     
       2. The ferritic stainless steel according to  claim 1 , wherein in the chemical composition, Cu content is 0.01% to 0.5%. 
     
     
       3. The ferritic stainless steel according to  claim 2 ,
 wherein in the chemical composition, Mo content is 0.01% to 0.3%. 
 
     
     
       4. The ferritic stainless steel according to  claim 1 ,
 wherein in the chemical composition, Mo content is 0.01% to 0.3%. 
 
     
     
       5. The ferritic stainless steel according to  claim 1 ,
 wherein in the chemical composition, C content is 0.005 mass % to 0.030 mass %, Si content is 0.25 mass % or more and less than 0.40 mass %, and Mn content is 0.05 mass % to 0.35 mass %, 
 the volume fraction of the ferrite crystal grains is 5% or more and 30% or less, and 
 the ferritic stainless steel further comprises elongation after fracture in a direction orthogonal to a rolling direction is 28% or more, and a ridging height is 2.5 μm or less. 
 
     
     
       6. The ferritic stainless steel according to  claim 1 ,
 wherein in the chemical composition, C content is 0.005 mass % to 0.025 mass %, Si content is 0.05 mass % or more and less than 0.25 mass %, Mn content is 0.60 mass % to 0.90 mass %, and N content is 0.005 mass % to 0.025 mass %, 
 the volume fraction of the ferrite crystal grains is 5% or more and 20% or less, and 
 the ferritic stainless steel further comprises elongation after fracture in a direction orthogonal to a rolling direction is 30% or more, and a ridging height is 2.5 μm or less. 
 
     
     
       7. A process for producing the ferritic stainless steel according to  claim 1 , the process comprising:
 hot rolling a steel slab having the chemical composition according to  claim 1  into a hot rolled sheet; 
 performing hot-rolled sheet annealing by holding the hot rolled sheet at a temperature of 900° C. or more and 1050° C. or less for 5 seconds to 15 minutes, to form a hot-rolled and annealed sheet; 
 cold rolling the hot-rolled and annealed sheet into a cold rolled sheet; and 
 performing cold-rolled sheet annealing by holding the cold rolled sheet at a temperature of 800° C. or more and less than 900° C. for 5 seconds to 5 minutes. 
 
     
     
       8. A process for producing the ferritic stainless steel according to  claim 5 , the process comprising:
 hot rolling a steel slab having the chemical composition according to  claim 5  into a hot rolled sheet; 
 performing hot-rolled sheet annealing by holding the hot rolled sheet at a temperature of 940° C. or more and 1000° C. or less for 5 seconds to 15 minutes, to form a hot-rolled and annealed sheet; 
 cold rolling the hot-rolled and annealed sheet into a cold rolled sheet; and 
 performing cold-rolled sheet annealing by holding the cold rolled sheet at a temperature of 820° C. or more and less than 880° C. for 5 seconds to 5 minutes. 
 
     
     
       9. A process for producing the ferritic stainless steel according to  claim 6 , the process comprising:
 hot rolling a steel slab having the chemical composition according to  claim 6  into a hot rolled sheet; 
 performing hot-rolled sheet annealing by holding the hot rolled sheet at a temperature of 960° C. or more and 1050° C. or less for 5 seconds to 15 minutes, to form a hot-rolled and annealed sheet; 
 cold rolling the hot-rolled and annealed sheet into a cold rolled sheet; and 
 performing cold-rolled sheet annealing by holding the cold rolled sheet at a temperature of 820° C. or more and less than 880° C. for 5 seconds to 5 minutes.